Detailed losses

11.5 Detailed losses

Users can switch and set various types of loss values based on the "Loss type" option displayed on the interface.

11.5.1 Thermal loss factor

In the "Thermal loss, Ohmic loss" section, users can set values for "Thermal loss factor", "DC side ohmic loss", "Low AC side ohmic loss", "MV Side", and "HV side" losses.

1. Thermal loss factor

There are currently various thermal models for PV modules, such as empirical fitting formulas based on outdoor measured data or calculations using heat conduction theory. The software, like PVsyst software, also uses a thermal model to calculate the temperature of modules. The two key parameters used to characterize heat loss are "Uc" and "Uv". "Uc" is a constant, and "Uv" is a variable related to wind speed. There are three default installation methods within the software, corresponding to different "Uc" and "Uv" experience data for users to choose from.

The software simulates in hours, and at each simulation time point, the thermal loss of the array is calculated based on the thermal equilibrium to determine the instantaneous operating temperature of the PV module. The formula involved in thermal equilibrium is:

Thermal loss factor: U = Uc + Uv * wind speed, unit: [W/m ² · K]

In many cases, due to the lack of collected wind speed data in the field area, the software will merge the "Uv" parameter into "Uc" with the default wind speed above the array of 1.5m/s (or the wind speed at the meteorological station of 3.3m/s), thus providing three default heat loss parameter settings.

The three default configurations are differentiated based on the ventilation conditions behind the array.

①Free standing system: Uc=29W/m²·K, Uv=0W/m²·K/m/s (where 29W/m²·K means that for every 1℃ increase in temperature, an additional irradiance of 29W/m² is required;W/m²·K/m/s increases the influence of wind speed on the former).

②Semi-integration: Uc=20W/m²·K, Uv=0 W/m²·K/m/s.

③Fully insulated backside: Uc=15W/m²·K, Uv=0W/m²·K/m/s.

2. DC side ohmic loss/Low AC side ohmic loss

The electricity generated by the PV array needs to be transmitted to the inverter through DC cables. The inverter converts DC electricity into AC electricity, which is then connected to the grid through one or more boosting processes. During this process, DC side ohmic losses, Low AC side ohmic losses, and transformer losses can all be configured in the software.

There are two calculation methods for DC side ohmic losses and Low AC side ohmic losses: "According to model" or "Specified value at STC". If the unconnected model is used, the "According to model" calculation method cannot be applied.

Tips： Whether choosing "According to model" or "Specified value at STC" type for calculation, the software's configuration of DC side ohmic loss and low AC side ohmic loss is based on the settings under STC operating conditions. In fact, cable ohmic loss is directly proportional to the array of the current. Therefore, compared to the STC standard irradiance of 1000W/m ², if operated at half the irradiance (i.e. 500W/m ²), the relative loss will only be one fourth. The actual ohmic loss for the whole year will be presented in the simulation results.

3. Setting of medium voltage side and high voltage side

When the check boxes on the left of "MV side" and "HV side" are selected, the software can set the relevant losses of the medium voltage transformer and the high voltage transformer separately.

The main losses of transformers are iron loss and copper loss, among which iron loss is the self loss of transformers during no-load operation (without load), consisting of hysteresis and eddy current losses, approximately equal to no-load loss. The value mainly depends on factors such as iron core silicon steel sheets, frequency, and magnetic flux density. Iron loss is the inherent loss of transformers, approximately proportional to magnetic flux density and primary side voltage, and is independent of load size. The software defaults iron loss to 0.1% of the rated capacity of transformers.

The copper loss of a transformer is the power loss caused by the current passing through the primary and secondary winding resistors when there is a load in a PV power generation system. Because power is equal to the array of current multiplied by resistance, the magnitude of copper loss is proportional to the array of primary and secondary currents. The software defaults the iron loss to 1% of the rated capacity of the transformer.

Tips： If the model is a non connected model, the default capacity ratio is 1.3, and the rated capacity of the transformer is the DC side capacity/1.3.

Even if the PV system stops generating electricity at night, the problem of transformer iron loss still exists. If users want to avoid operating transformers at night, they can choose to check the "Night disconnect" option.

11.5.2 Module quality,Mismatch,Soiling

In the "Module quality,Mismatch,Soiling" section, users can set the loss values for "Module quality LID Loss", "Mismatch loss", "Modules degradation" and "Soiling loss yearly".

1. Module quality LID Loss

①"Module quality": There may be a certain deviation between the actual power and nominal power of the module. For conventional modules, the manufacturer's promised deviation is generally within 0~+5W. The impact of this power deviation can be set according to the promised actual power deviation value. The default value in the software is "-0.4%", and it is important to note the meaning of positive and negative values. Setting this parameter to a negative value indicates that the module power has a positive deviation.

②"LID loss": The attenuation rate of PV modules refers to the ratio of the maximum output power under standard test conditions (AM1.5, module temperature 25 ℃, irradiance 1000W/m ²) after a period of operation (several days or more) to the maximum output power during initial production and operation. Normally, PV modules undergo a significant power drop during the initial usage phase, known as the initial photoinduced attenuation stage. Afterwards, the trend of power decline will become slower.

2. Mismatch loss

The "Mismatch losses" mainly stems from the inconsistency of module voltage and current, as well as the voltage drop difference between the string and the inverter. Usually, the impact of current mismatch is significantly greater than that of voltage mismatch.

3. Modules degradation

The "Modules degradation" loss of a module refers to the performance parameter that decreases year by year over time. For example, assuming the linear rate of the module is 0.4%, the degradation loss at the beginning of the year is 0%, and at the end of the year it reaches 0.4%. Due to the linear and uniform occurrence of degradation, the average degradation loss in the "Loss diagram" table is: (0%+0.4%)/2=0.2%.

Tips： Usually, module manufacturers declare the initial annual degradation rate and subsequent annual linear degradation rate of the modules in their product manuals. Taking a N-type module as an example, the manufacturer promises a degradation rate of 1% in the first year and a linear decay rate of 0.4% annually thereafter. Based on this, the LID loss is calculated as 1% -0.4%/2=0.8%, while the annual linear degradation is 0.4%. In the"Loss diagram" table of the report, the LID loss is 0.8%, while the annual linear loss is 0.2%.

4. Soiling loss yearly

The impact of soiling on modules is mainly manifested in blocking light and affecting heat dissipation. Corrosive soiling may also corrode the surface of modules. The formation of soiling is related to various factors such as air quality, rainfall, module inclination angle, and module cleaning frequency. Therefore, it is quite difficult to choose an exact value when evaluating the losses caused by soiling.

The current standard value for PV power system is set at 3%, and users can also adjust based on specific factors such as monthly air conditions, rainfall, climate conditions, and cleaning cycles by clicking on "Set monthly value".

Tips： The soiling loss can also be used to describe the impact of winter snow cover on PV modules. For example, setting a loss rate of 50% means that there are 15 days in the winter month when PV modules are covered by snow.

11.5.3 Other losses

In "Other losses", users can set values for "IAM loss", "Auxiliaries losses", "Shading loss", and "Unavailability loss".

1. IAM loss

IAM (Incident Angle Modifier) is related to the reflection ability of optical materials, incident angle, and packaging glass material of PV modules.

In the software, we considered three different calculation methods: firstly, based on the "*.PAN" file model, secondly, based on the characteristics of Fresnel normal glass ("Fresnel, normal glass"), and finally based on the characteristics of Fresnel AR coating ("Fresnel AR coating"). Choosing different models may result in different IAM losses, depending on meteorological data and module installation tilts.

2. Auxiliaries losses

The "Auxiliaries losses" refers to the loss of electricity used by the station. When calculating 8760 hours of power generation, "Constant power loss" refers to the fixed power consumption of the inverter when outputting electricity. "Consumption proportional to the system power delivery" is usually used to set the energy required for inverter cooling, which is proportional to the output power of the inverter. "Night consumption" usually refers to the loss of electricity used by the station at night, but does not include the night time loss of the inverter itself.

3. Shading loss

"Shading loss" refers to the quantification of energy loss caused by shading when calculating 8760 hours of power generation.

In the process of estimating 8760 hours of power generation, the software first calculates the optical shading based on the relative positions of each array in the model. Subsequently, it will use a specific math model to reflect the impact of optical shading in the output power of the PV string, namely the "Near Shadings". The software provides two modes for selection: "Linear" and "By string".

"Linear": This mode assumes that the power generation loss of modules is linearly related to the shading ratio, and only considers the reduction of irradiance, so the calculated shading loss is relatively low. In the generated calculation report, the energy loss flow table will only display the loss of "Near Shadings: Irradiance Loss".

"By string": Divide the array into several strings, and if there is shading in each string area, the power generation efficiency directly radiated from that area will be affected accordingly. Specifically, an "Fraction for elec. effect" parameter should be defined. This factor needs to be roughly estimated based on experience: For scattered shading object, such as chimneys, distant buildings, etc., usually 60% -80% is selected; For more regular shading object (such as shadows between the front and rear rows of arrays), a higher factor should be set. In the generated calculation report, the energy loss flow table will not only display the "Near Shadings: Irradiance Loss" but also the "Shadings: Electrical Loss acc. to Strings".

For flexible support or ditch projects, usually a array does not form a complete string. Therefore, when creating a array, it is necessary to check the checkbox for "Determine at confluence". At this point, the modules need to be combined into a string before 8760 hours of power generation can be calculated. For a half string array, it is also necessary to pair the half string array into a whole string before calculating the 8760 hour power generation.

4. Unavailability loss

The "Unavailability loss" refers to the time loss caused by the inability of a PV power station to generate electricity due to malfunctions or regular maintenance. It should be noted that the ratio set is based on time, not power generation. It corresponds to the number of days set below. For example, if the loss value is set to 2%, then in a year of 365 days, 2% is equivalent to 7.3 days (175 hours) of failure duration. The "Beginning Date / Hour " and "Duration" are set to three time periods, allowing users to set specific hours for each time period.